1. Field of Invention
This invention is in the field of electromagnetic interference reduction and methods therefore and, more particularly, is a phase modulator for spreading the spectrum of radiated emission of a clock signal and method therefor.
2. Description of the Prior Art
Components of a digital computer, such as a microprocessor, are called upon to operate in synchronism with clock signals of increased frequency as a result of a demand for increased speed of operation of the computer. The increased clock frequency causes an increase of electromagnetic radiation from clock signal lines and all computer signal lines controlled therefrom.
The radiation may cause an undesired coupling of the clock and its related signals to devices that are in proximity to the clock and computer signal lines. The coupling has been reduced by spreading the spectrum of the clock signal and thereby spreading the spectrum of electromagnetic radiation caused by the clock signal. The spreading of the spectrum results in a reduction in the average value of the radiation.
The source of the clock signal has been frequency modulated to provide a frequency modulated clock signal comprised of a multiplicity of frequencies within a spread spectrum. Because the spreading of the spectrum results in the reduction in the average value of the radiation, there is a corresponding reduction in the undesired coupling.
The use of the frequency modulation for spreading the spectrum of a clock signal is described in "Spread Spectrum Clock Generation for the Reduction of Radiated Emissions" by K. B. Hardin, J. T. Fessler and D. R. Bush in a paper submitted to the IEEE 1994 International Symposium on Electromagnetic Compatibility, Aug. 22-26 1994 and in an article entitled "Digital Circuit Radiated Emission Suppression With Spread Spectrum Techniques" in the 1994 issue of ITEM by the same authors. The article states in a reference note that there is a pending patent application relating to the subject matter therein.
Although the use of frequency modulation to spread the spectrum is effective, it is predicated upon access to the source of the clock signal. In an existing system, the access to the source of the clock signal may be impractical.